It is known in the art to prepare free standing, pinhole-free ultrathin films suitable for separatory applications. In U.S. Pat. No. 4,880,699 to Kohn there is disclosed and claimed, for example, a method of preparing thin polyimide films. Such films are prepared by dissolving the polymer in a solvent, casting the solution on water, and evaporating the solvent. The film so formed can then be readily removed from the water's surface.
A free standing film fabricated in accordance with the foregoing may be difficult to incorporate into commercially attractive structures, for example, into a hollow fiber membrane module useful for gas separation. One alternative approach might be to dip-coat hollow microporous fibers. Such a method, however, would undoubtedly have the disadvantage that the polymer solution would penetrate the microporous structure due to the wicking effect, leading to non-uniform thicknesses of the coating. Another possibility for applying thin, continuous cast membranes to a hollow fiber would be mechanical wrapping. This method is arduous at best, and coating thicknesses and film quality would largely be determined in the casting step. 0 In Japanese laid-open application No. 63-104618 of Kawada et al. there is disclosed a method of producing hollow fiber composite membranes. The method shown involves continuously casting a thin film on water and passing a microporous hollow fiber through the polymer solution in a region where the polymer/solvent solution possesses fluidity. The solution deposits a thin film on the membrane while excess polymer is taken up and stored. The system utilized by Kawada et al. does not address the issues of depositing a thin film uniformly about the periphery of a hollow fiber as would be required for high quality separation membranes. Moreover, significant control and recycling of polymer would be required, making the system difficult to automate or even produce commercially suitable product.